Description is made hereinafter for an example of a conventional type of microsensor with reference to a temperature sensor making use of changes in thermal conduction. This temperature sensor makes use of a fact that thermal conductivity of a atmospheric gas changes according to a density of vapor (humidity). In the temperature sensor, two thin film resistor heat generating chips each having the same characteristics are used, and one of the chips is sealed with a particular atmosphere such as air having a certain degree of humidity and used as a chip for temperature compensation, while the other chip is located in a space where the atmosphere can circulated to and from the outside and is used as a chip for detection.
With the configuration as described above, the chip for temperature detection is located in the atmosphere having the atmospheric thermal conduction characteristics corresponding to a humidity which is equal to that of the external air, and when a certain quantity of heat is generated in the chip for temperature detection, the temperature goes up to a certain level where the temperature is balanced with a heat capacity corresponding to the thermal conductivity of the space in which the chip for temperature detection is installed. For this reason, the chip for detection enters a state, according to the temperature coefficient, in which the chip has a resistance value corresponding to a temperature in the space. If a heat value of the chip for temperature compensation is kept at the level as that of the chip for temperature detection in this state, a temperature difference corresponding to a difference of absolute humidity in the spaces where the two chips are installed respectively, accordingly a difference between resistance values of the two chips is generated, and an absolute temperature of the external air can be measured by providing the difference between the resistance values as an output indicating imbalance in a bridge circuit.
In this film resistor heat generating chip, temperature changes according to change of a temperature of external air, but in that case also the same temperature change occurs in both the chip for temperature compensation and the chip for temperature detection, so that as a principle output from the bridge circuit described above does not change, and it is possible to detect only the absolute temperature irrespective of the external air temperature.
In a case where characteristics of the sensor is affected by temperature of the working environment for use of the sensor, it is necessary to subtract fluctuation depending on temperature change from the measured temperature using a principle of comparing output data from the temperature compensating section to that from the temperature detecting section. If the temperature compensating section and the temperature detecting section are very close to each other, it is possible to accurately detect a temperature around the detecting section. For this reason, the temperature compensating section and the temperature detecting section should be located at positions close to each other on the same substrate.
FIG. 3 is a cross sectional view illustrating an example of a sensor chip 300 having the configuration as described above, and in this figure, designated at the reference numeral 301 a substrate, at 302 an insulating protective film mounted on the substrate 301, at 303 a heat generating section located on the insulation protective film 302, at 304 a temperature compensating section, at 305 a humidity detecting section, at 306 a air hole provided in the temperature detecting section 305, and at 307 an electrode connected to the heat generating section 303.
FIG. 4 and FIG. 5 are drawings each illustrating a state where the sensor chip 300 having the configuration as show in FIG. 3 has been assembled in a casing, and in the figures , at the reference numeral 401 is designated at a base section, at 402 a lead pin, at 403 a nickel cap which is engaged with the base section 401, at 404 a stainless duplex mesh, at 405 a bonding wire used for connection between the sensor chip 300 and the lead pin 402, at 406 die bond adhesive used to make the sensor chip 300 adhere to the base section 401.
In the configuration described above, generally the sensor chip 300 shown in FIG. 3 has been assembled in a casing and a microsensor has been driven by providing pulse drive control in an energized state with 50 msec pulse width via a power supply unit 601 from the Wheatstone bridge circuit shown in FIG. 6.
In the conventional type of microsensor as described above, however, the sensor is intermittently controlled according to pulse drive, so that in some operating environments the electrode section is sometimes shortcircuited due to dew condensation, as a result output voltage is not obtained, and the functions as a sensor is lost.
Description is made hereinafter for the above-described problem with reference to the related drawings. The state where the peripheral environmental conditions detected by the microsensor are as shown by the graph in FIG. 7 (relative humidity/elapsed time) is assumed. A microsensor has a minute construction and its heat capacity is very small, so that the microsensor is used under the conditions, for instance, that 7 V power is loaded for 1 sec at a pulse rate of once for 50 msec, but when the elapsed time passes over T1, the humidity raises to more than 90% and the subsequent output voltage can not be obtained. This phenomenon occurs because the electrodes are electrically shortcircuited due to dew condensation generated between the electrodes shown in FIG. 3.